Same as others, in my 2 completed Mistlands games we've found seals faster than all cores...

It really does seem like RNG unluck. I usually struggle the most with boss stones of all things haha.

If you have boss uncovered, just go there and run around a bit, there should be plenty mines. If you put some effort in efficiency in it should not take long to find the rest of them. Or if the group does not want to play, just spawn them and have everyone do the boss.